When one mentions "Brightness" and "Contrast" what immediately comes to mind are the familiar controls that are commonly found on television receivers. In television, a brightness adjustment changes the dc level (i.e., "offset") of the overall television signal, and changing the contrast setting causes an increase or decrease in the range between the least illuminated and most illuminated portions of the picture (i.e., "gain"). In a modern digital oscilloscope, the brightness and contrast controls produce a somewhat similar effect but operate in a very different manner on very different signals.
A brief discussion of the operation of a modern digital oscilloscope may be in order before proceeding to the subject invention. Modern digital oscilloscopes generally use raster scan displays to present the activity of electrical signals to their users. Each raster scan display, such as those seen every day on computer screens, consists of a two dimensional array of pixels, with each pixel location being uniquely defined by a row number and column number. The simplest and lowest cost versions of such displays are "single bit" displays, in that the memory from which they derive the information to be displayed only has one bit of intensity information associated with each pixel. In such a display, that single bit of information determines whether the pixel associated with it is either "on" or "off", with "on" dictating that a predetermined amount of intensity is to be used to illuminate the pixel and "off" indicating that the pixel is not to be illuminated at all.
The more complex and expensive alternative to a single bit display is a multi-bit display, which can provide variable intensity (also known as "gray-scale") or color variations as a substitute indicator of brightness. The memory locations associated with each pixel of a variable intensity display contain multiple bits of intensity information, indicating the number of varying intensity levels with which they can be illuminated. Like the pixels of single bit displays, those of multi-bit displays can exhibit an "off" or dark state, but instead of one value of illumination, they have multiple values. Typically, the number of values available is 2N-1, where N is the memory depth at each address of the raster memory. Thus, for example, a four bit deep raster scan memory can support fifteen levels of partial through maximum illumination, as well as the dark or "off" state. Pixel intensity can also be translated into differing colors, as well as intensity or "brightness".
With this larger amount of data, multi-bit displays can convey more information about the behavior of electrical signal waveforms under observation, particularly if the signal is not perfectly repetitive and therefore has less activity in some portions than others. U.S. Pat. No. 4,940,931 "Digital Waveform Measuring Apparatus Having A Shading-tone Display" (Katayama et al.) describes a system for producing digital variable intensity displays.
Typically, digital oscilloscopes acquire information about the behavior of a circuit node by periodically sampling the voltage present at the node. The oscilloscope probe tip is placed in contact with the node and the probe and front end of the oscilloscope precisely replicate the signal, or some predetermined fraction or multiple of the signal, and present it to an analog-to-digital converter. The output of the analog-to-digital converter is a series of multi-bit digital words that are stored in an acquisition memory. Successively acquired samples are stored at sequentially related addresses in the acquisition memory, and are thereby related to a time scale. Those addresses will eventually be converted back to a time scale, one of which is represented as horizontal distance along the x-axis of the oscilloscope's raster scan display.
In a typical digital oscilloscope, voltage amplitude values derived from the data contents of an acquisition memory location determine the vertical location (row number) of an illuminated pixel, while time values derived from the addresses of the acquisition memory determine the horizontal location (column number). The process of expanding the contents and addresses of an acquisition memory to produce contents for a two dimensional raster memory is known as "rasterization".
The output of a rasterization process is usually combined with some preexisting content of a raster memory, and the resulting composite raster contents may thereafter be regularly subjected to some sort of decay process. Digital persistence and decay are known, for example, from U.S. Pat. No. 5,440,676 "Raster Scan Waveform Display Rasterizer With Pixel Intensity Gradation" (Alappat, et al.), U.S. Pat. No. 5,387,896 "Rasterscan Display With Adaptive Decay" (Alappat, et al.), and U.S. Pat. No. 5,254,983 "Digitally Synthesized Gray Scale For Raster Scan Oscilloscope Displays" (Long, et al.).
The result of the above described process is a raster memory comprising memory locations each of which contains a multibit digital word representative of the desired illumination of a corresponding pixel of the display.
U.S. patent application Ser. No. 09/056,042, "BITS-PER-PIXEL REDUCTION FROM VARIABLE INTENSITY RASTERIZER TO VARIABLE INTENSITY OR COLOR DISPLAY" (Siegel, et al.), filed Apr. 3, 1998 and assigned to the same assignee as the present application, discloses the operation of contrast and brightness controls in an oscilloscope such as the one described above. In accordance with that invention the bits-per-pixel values are decreased under control that permits the operator to choose the gain and offset of the transfer function. From the user's point of view such controls appear, respectively, as contrast and brightness adjustments. It is disclosed in that document that a 21-bit pixel intensity value be mapped into a 4 bit pixel intensity display word by comparing the 21-bit value against a series of breakpoints, and setting the 4 bits of the pixel intensity display word accordingly. It is further disclosed in that reference that, in one embodiment, the maximum pixel intensity value can be set via a brightness control.
The TDS-7xxD series oscilloscopes manufactured by Tektronix Inc. are actual oscilloscopes embodying the teaching of Siegel, et al. In the TDS-7xxD the contrast and brightness functions are controlled via a single knob which is assigned to each function by means of a menu choice. From the user's point of view, control of these functions is essentially the same as having two knobs, one for brightness and one for contrast. Operation of these controls will be explained below in comparison to the operation of the subject invention.
While this arrangement performs adequately, it is felt that the alternate use of the same knob for the two different functions may lead to some consumer confusion, especially since the contrast and brightness functions are somewhat interactive, as will be explained below. Also, for reasons to be explained below, it was found that when the display was switched from color to black and white, or vice versa, that the contrast and brightness settings had to be readjusted.
What is needed is an apparatus and method for quickly and easily controlling both brightness and contrast simultaneously via a single knob, producing an output which makes good use of the dynamic range available, and causing that setting to behave in an equivalent manner in both black and white and color modes of operation.